Laboratory fume hoods are generally classified into two types. One type is the horizontal sash fume hood, such as shown in U.S. Pat. No. 2,715,359; and the other type is the vertical sash fume hood, such as shown in U.S. Pat. No. 3,747,504.
Horizontal sash fume hoods have a distinct advantage in that the two sliding doors, such as 26 and 26' in FIG. 1 of U.S. Pat. No. 2,715,359, prevent all the sashes from being open at the same time. Thus, a wide horizontal workbench area can be provided for laboratory experiments to be set up, but only the immediate area being used is open for operator access. This conserves the heated and cooled room air and prevents excess room air from being sucked out through the fume hood.
In a single vertical sash fume hood, such as in U.S. Pat. No. 3,747,504, an access opening to the entire work area is completely open when the sash is raised. This can cause excessive loss of room air since generally only a portion of the laboratory bench is used at one time.
To cut down loss of heated and cooled room air, a proposal has been made for a hybrid type fume hood that includes both a vertical and a horizontal sash as disclosed in U.S. Pat. No. 4,142,458. However, such a system is more complicated because it requires a complete horizontal tracking and roller system, as well as a vertical tracking system for the vertical sash. Since the vertical sash and horizontal sash are of different sizes and constructions and are not interchangeable, the cost of such a hybrid sash system would be much more than either a conventional vertical sash or horizontal sash type fume hood.